Covered rolls are utilized in many applications including an application known as calendering. Calendering is the act of pressing a material, e.g., cloth, rubber or paper, between rollers or plates in order to smooth the material into sheets. Covered rolls are precision elements and therefore must be precisely manufactured to achieve specific size and shape specifications with tight tolerances. For many applications, covered rolls must also have high temperature and pressure resistance. For example, covered rolls are used in paper mills for calendering paper-web material into specific grades of paper, often under severe conditions.
Prior art calender rolls comprise a metal cylinder to which a synthetic composite roll cover is added to preclude metal-on-rolled-material-on-metal contact at the nip between calender rolls during a calendering operation. Most of these synthetic composite roll covers use some form of extrudable thermoplastic material or a thermoset resin such as epoxy, silicone rubber or polyurethane as a base material, which is combined with some form of reinforcement material to improve strength.
More recently, roll covers have been fabricated out of non-composite thermoplastic and thermoset materials. Many thermoset materials in particular have highly desirable properties making them superior to synthetic composite materials. These properties include higher tensile strength, greater smoothness, higher impact strength, more uniform surface finish and more homogenous physical and thermal properties.
U.S. Pat. No. 6,206,994 discloses a method and apparatus for the fabrication of rolls covered with high performance thermoplastic materials. The disclosed apparatus comprises a mold tape dispenser and a thermoplastic extruder located on a turntable. In operation, a roll core, vertically oriented, is lowered through the center of the turntable. As the roll core is lowered, the turntable rotates concentrically around the non-rotational roll core. The rotation of the turntable winds tape helically around the lowering roll core, the tape being spaced a predetermined distance from the roll core. Between the tape and the roll core, the turntable rotation winds an extruded thermoplastic filament helically onto the role core. When the entire roll core has been lowered through the turntable, the roll core is covered with thermoplastic filament.
The above described patent discloses a large and unwieldy apparatus with numerous moving components. During operation of the apparatus, a substantial proportion of the apparatus is in motion: the roll core must be lowered and raised a distance equivalent to at least the length of the roll core; and all of the thermoplastic material application equipment must be rotated around the roll core. Besides being difficult to manufacture and maintain, the moving components of the apparatus also consume considerable energy.
The apparatus disclosed by Pat. No. 6,206,994 also occupies considerable space. To accommodate the apparatus, a space with a height at least two times the length of a roll core is required.
In light of the above, there is a need for a less complicated method and apparatus, with greater space efficiency and energy efficiency, for covering a roll core with a polymeric material.